1. Field of the Invention
The present invention relates to a polymer electrolyte membrane and a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
In recent years, a polymer electrolyte fuel cell is required to be operated in higher temperature and lower humidity environment especially for automobiles. Accordingly, as a proton conductive polymer contained in a polymer electrolyte membrane and a catalyst layer of a membrane/electrode assembly, a material exhibiting high electrical conductivity in a low humidity environment has been desired.
To improve electrical conductivity of a proton conductive polymer, the number of ionic groups should be increased. However, an increase of ionic groups increases the water content of the proton conductive polymer. If the water content of the proton conductive polymer to be used for a polymer electrolyte membrane is too high, the proton conductive polymer will swell, whereby the volume of the polymer electrolyte membrane will increase. As a result, the membrane strength of the polymer electrolyte membrane will decrease, whereby there is a concern that the durability will decrease during operation for a long period of time.
As a material for the polymer electrolyte membrane, a fluoropolymer is usually employed from the viewpoint of chemical durability. As such a polymer, the following polymers are, for example, known.
(1) A copolymer of tetrafluoroethylene with CF2═CF—(OCF2CFX1)m—Oq—(CF2)n-A (wherein X1 is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, n is an integer of from 1 to 12, q is 0 or 1, and A is a sulfonic acid type functional group) (Patent Document 1).
(2) A copolymer represented by —[CF2—CF2]k—[CF2—CF(—O(CF2)2—SO3H)]1— (Non-Patent Document 1).
The polymer (1) tends to be a polymer having a very high water content in order to maintain high electrical conductivity in a low humidity environment, whereby there is concern that the durability of the polymer electrolyte membrane will decrease during operation for a long period of time.
The polymer (2) has insufficient electrical conductivity, and if the ion exchange capacity of the polymer is increased in order to increase the electrical conductivity, the water content tends to be too high, and there will be concern that the durability of the polymer electrolyte membrane will decrease during operation for a long period of time.
Patent Document 1: JP-A-6-260185
Non-Patent Document 1: Masanori Ikeda et al. Polymer Preprints, Japan, 2005, vol. 54, No. 2, p. 4,521-4,522